Disc drive devices have become readily accepted as computer peripheral devices for long term storage of large quantities of data. Such devices typically are constructed either as rigid disc drives, also known as hard or Winchester disc drives, or "floppy" disc drives. Hard disc drive devices offer storage capacities that are several orders of magnitude greater than floppy disc drive devices and typically contain one, two or more hard discs that generally are permanently installed. Floppy disc drive devices, on the other hand, allow the user to load and unload the floppy disc, thus enabling the user to store different types of information on different discs, as needed. Although the present invention is available for use in floppy disc drive systems, it is primarily intended for use in hard disc drive devices in which the head for reading a disc rests against the disc during off-power conditions.
In such hard disc drive devices, when the power is on and the disc is rotating, the head (or heads, as typically plural surfaces of plural discs are used for the writing and reading of data) "flies" over the surface of the disc with the rotation of the disc creating an air bearing on which the head is carried. If this air bearing is reduced, as when the rotating disc slows, the head may crash to the surface of the disc, thus resulting in damage to the disc and to the head and, moreover, resulting in significant loss of recorded information due to the crash. To avoid significant changes in rotary speed of the disc, the disc drive spindle motor which rotatably drives the disc is controlled to detect and account for speed variations.
Notwithstanding desired spindle motor speed controls, additional precautions must be taken when the disc drive is deenergized to avoid the destruction of recorded data. Conventionally, hard discs are provided with an information free zone in which no data is recorded and upon which the head is intended to rest during power-off conditions. Therefore, when the usual power switch is opened to deenergize the disc drive, circuitry might be triggered, as under microprocessor control, to retract the head to the information-free zone before power is actually interrupted. That is, the usual head positioning apparatus, such as a stepper motor or a voice coil drive motor, both of which are referred to generically as the head actuator, is controlled to move the heads to a "home" or "park" position, i.e. any position within the zone that is safely away from the usable surface of the disc. If the head is moved in time to this retracted home position, there is no danger of it destroying useful data when it lands on the surface of the disc after the termination of rotation following power interruption. Advantageous apparatus for retracting the head and braking the spindle motor in such a disc drive device is disclosed in U.S. Pat. No. 4,658,308 (Sander, Jr.), assigned in common with the present invention. This apparatus uses the emf that is generated by the disc drive spindle motor as a result of the rotation of that motor following a power interruption to retract the head to the home position.
Once the head has been retracted to the information-free zone and has come to rest thereon, the head thereafter should be held at its retracted position while the power remains off. Otherwise jostling or shaking of the device, for example while the device is being moved from one location to another, could cause the head to move away from the zone into an area of the disc on which information is recorded and, since the disc is not then rotating and the head is not "flying," information could be destroyed and both the head and disc damaged. A variety of mechanical/electrical structures have been proposed for moving the head assembly of a disc drive to its retracted position and holding it there during power off times. In such structures, the mechanism for holding the head assembly should be as simple and yet as foolproof as possible. It also should be small and lightweight, an advantageous feature in disc drives considering the move towards miniaturization. To this end, large and powerful springs are bulky and heavy and should be avoided if possible. The mechanism should also be foolproof since, particularly in a hard disc drive, the discs are permanently encapsulated within the apparatus and damage to either a disc or a head can require significant repair or replacement.